1. Field of the Invention
This invention relates generally to a system and method for reducing catalyst degradation in the MEAs of a fuel cell stack at system start-up and, more particularly, to a system and method for reducing cathode catalyst degradation in the MEAs of a fuel cell stack at system shutdown and start-up that includes determining an anode purge velocity and time depending on how long the stack has been shutdown.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include two hundred or more fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
When a fuel cell system is shut down, un-reacted hydrogen gas remains in the anode side of the fuel cell stack. This hydrogen gas is able to diffuse through or cross over the membrane and react with the oxygen in the cathode side. As the hydrogen gas diffuses to the cathode side, the total pressure on the anode side of the stack is reduced below ambient pressure. This pressure differential draws air from ambient into the anode side of the stack. When the air enters the anode side of the stack it generates an air/hydrogen front that creates a short circuit in the anode side, resulting in a lateral flow of hydrogen ions from the hydrogen flooded portion of the anode side to the air-flooded portion of the anode side. This high ion current combined with the high lateral ionic resistance of the membrane produces a significant lateral potential drop (˜0.5 V) across the membrane. This produces a local high potential between the cathode side opposite the air-filled portion of the anode side and adjacent to the electrolyte that drives rapid carbon corrosion, and causes the carbon layer to get thinner. This decreases the support for the catalyst particles, which decreases the performance of the fuel cell.
It is known in the art to purge the hydrogen gas out of the anode side of the fuel cell stack at system shutdown by forcing air from the compressor into the anode side at high pressure. The air purge also creates an air/hydrogen front that causes the cathode carbon corrosion, as discussed above. Thus, it is desirable to reduce the air/hydrogen front residence time to be as short as possible, where the front residence time is defined as the anode flow channel volume divided by the air purge flow rate. Higher purge rates will decrease the front residence time for a fixed anode flow channel volume.
It is also known in the art to provide cathode re-circulation to reduce cathode corrosion at system shutdown. Particularly, it is known to pump a mixture of air and a small amount of hydrogen through the cathode side of the stack at system shut-down so that the hydrogen and oxygen combine in the cathode side to reduce the amount of oxygen, and thus the potential that causes the carbon corrosion.
It is also known to short circuit the stack with a resistor at system shut-down to reduce the amount of oxygen on the cathode side of the stack, and thus cathode side corrosion. It has been shown that these two techniques do provide mitigation of carbon corrosion on the cathode side of the stack. However, improvements can be made.
At the next system start-up, assuming enough time has gone by, both the cathode and anode flow channels are generally filled with air. When hydrogen is introduced into the anode flow channels at system start-up, the hydrogen pushes out the air in the anode flow channels also creating a hydrogen/air front that travels through the anode flow channels. The hydrogen/air front causes a catalytic reaction along the length of the membrane in each fuel cell as the front moves that, in combination with the reaction across the membrane, creates a high electric voltage potential. This combined electric voltage potential is high enough to severely degrade the catalyst and the carbon particles on which the catalyst is formed, reducing the life of the MEAs in the fuel cell stack. Particularly, the reaction created by the hydrogen/air front in combination with the normal fuel cell reaction is orders of magnitude greater than only the fuel cell reaction across the membrane. For example, it has been shown that without addressing the degradation effects of the hydrogen-air front at system start-up, it only takes about 100 shutdown and start-up cycles to destroy the fuel cell stack in this manner.
It has been proposed in the art to reduce the degradation effect of the hydrogen/air front at system start-up by forcing hydrogen through the anode flow channels as quickly as possible so as to reduce the time that the degradation occurs. It has also been suggested to introduce hydrogen into the anode flow channels at a slow rate to provide active mixing of the air and hydrogen to eliminate the hydrogen/air front. It has also been proposed in the art to cool the fuel cell before removing the hydrogen from the anode flow channels. However, all of these solutions have not reduced the hydrogen/air degradation enough to provide a desired lifetime of the fuel cell stack. Particularly, moving the hydrogen/air front quickly does not completely eliminate the degradation of the catalyst, and requires over-sized piping and other components to rapidly purge the air from the anode flow channels. Introducing the hydrogen slowly at start-up has the disadvantage of requiring a recirculation pump that takes several minutes to completely remove the air from the anode flow channels. Further, requiring precise control of the amount of hydrogen into the anode flow channels is difficult to implement.
It has also been proposed in the art to replace the materials in the MEAs so that the carbon is less susceptible to the hydrogen-air reaction. One example is to use graphitized carbon. However, there are certain problems with this solution that reduce the performance of the fuel cell stack.
It has also been proposed in the art to provide a load across the fuel cell stack, such as a resistor, to reduce the electric potential generated by the hydrogen/air front. However, an extremely low resistance load will require electrical components with a high power rating. Also, flow and balancing between cells in a fuel cell stack can result in corrosion to the cell anodes. Furthermore, in most embodiments, a resistor is typically not sufficient on its own to minimize carbon corrosion.